1. Field
The present invention relates to a semiconductor device including a circuit board obtained by bonding metal plates to both sides of an insulating substrate respectively, at least one semiconductor element bonded to an external surface of one of the metal plates through a first solder, and a radiating base plate bonded to an external surface of the other metal plate through a second solder.
2. Description of the Related Art
The semiconductor device, for example, a power module to be used in a motor driving control system is required to have a sufficient insulating property and radiating property with an increase in a power. For this reason, an insulating substrate formed of ceramic is used to bond a power semiconductor element to a first copper plate bonded to one of surfaces through a lead based solder having a high melting temperature and to subsequently bond a radiating base plate to a second copper plate bonded to the other surface of the insulating substrate through a tin-lead based eutectic solder having a comparatively low melting temperature in order to prevent a bonded portion of the lead based solder from being molten again.
On the other hand, in recent years, a solder material is advanced to be lead-free in order to achieve an environmental protection. The lead-free solder has a melting temperature which is higher than that of the tin-lead based eutectic solder and is lower than that of the lead based solder. For this reason, there is a problem in that a solder bonding portion on an element side bonded earlier is molten again by a heat in a processing for bonding a radiating base plate, resulting in a remarkable deterioration in a bonding reliability if solder bonding is carried out by the conventional technique using the lead-free solder.
Moreover, the lead-free solder is harder than the lead based or tin-lead based solder. For this reason, there is a problem in that a crack is developed rapidly if it is once generated, resulting in a deterioration in an endurance.
Therefore, there are known a technique for bonding a semiconductor element to a radiating base plate by using two types of lead-free solders having different melting temperatures from each other (for example, see JP-A-2006-237057) in order to solve the former problem, and a technique for adding a rare metal such as bismuth or indium to a lead-free solder to enhance an endurance (for example, see JP-A-2007-141948) in order to solve the latter problem. Furthermore, there is also known a special technique for considerably changing a structure of a power module and hardening a solder bonding portion with a mold resin to carry out resin sealing.
In the technique described in JP-A-2006-237057, however, it is necessary to specially select and prepare the two types of lead-free solders having the different melting temperatures from each other, and furthermore, to properly use the two types of solders for a bond of a semiconductor element to an insulating substrate and a bond of a radiating base plate. The handling is complicated as a whole. In the technique described in JP-A-2007-141948, moreover, it is necessary to specially use an expensive rare metal as an additive material to a lead-free solder. After all, both of the techniques have a problem in that a material cost or a managing cost suddenly rises, and furthermore, a manufacturing process is forced to be changed, resulting in a considerable increase in the cost.